Development of the locomotory muscle of the chaetognath Sagitta. 2. Stereological study of fibre growth and differentiation processes.

The cellular growth and differentiation of A and B fibers was studied using classical stereological methods and the results have then been analysed using linear regression analysis and hypothesis testing procedures. At a given level, the development of the muscular tissue slows down and stabilises, at this same level the growth of the two types of fibres slows down sharply. This is not the case for the cellular organelles. The growth of the contractile apparatus is continuous in each fibre, but the density of myofibrils is higher in the A fibres. The surface of the SR increases in a different fashion in the two types of fibre, but its volumic density evolves in a similar fashion. The mitochondria develop differently in the two types of fibre, neither their shape nor their distribution are comparable from one type of fibre to the other. Despite this overall difference between the mitochondrial populations the individual mitochondria growth mechanisms seem to be comparable. These parameters reflect the organisation and the development of the fiber groups and cellular architecture. They indicate the existence of unknown morphogenetic signals and fields, in this epithelial tissue having paracellular paths communicating with sea water. Muscle development seems to be largely a myogenic property regulated by various extrinsic factors, which are examined.